Accidentals and Alterations

Perhaps because it is more visually intuitive. I imagine you are referring to the vocal staff... but in the case of the bass staff, for example, F# is on the fourth line.

Maybe it is more justifiable there because that is where the clef rests.

...but thinking about those for Tenor, soprano...

It is certainly by convention; I don't know why

Giove wrote:

If I find a key signature alteration that turns all the Fs on the bottom line into F#, how should one behave with all the other Fs (I mean those between the lines and those above and below the staff)?

By convention, the key signature is marked like this: you alter one and it is understood that all are altered. Imagine if they had to mark them all, let alone above and below the staff! In C# major, you would have to mark more than 50 sharps!

Ok guys, but why that specific line?...that is also the point

Giove wrote:

Ok guys, but why that specific line?... that is also the point

Why does a red traffic light mean you have to stop and a green one means you must proceed?

The answer is: it is a convention.

Why are pedestrian crossings made of stripes and not squares?

The answer is: it is a convention.

Why is the flat sign shaped like a 'b' and the sharp sign like a hashtag and not like a 'd'?

The answer is: it is a convention.

Why is F# in the clef indicated on the 5th line?

The answer is: it is a convention.

Why do you ask yourself certain questions (the answers to which are useless, moreover)?

I don't know the answer to this question...

Giove wrote:

Ok guys, but why that specific line?... that is also the point

It is a convention, but one created for a reason.

Carlos wrote:

Why does the red traffic light mean you have to stop and the green one means you must proceed?

Because the color red has always been (perhaps recalling the color of blood) the color of danger, while green is its complementary color.

The 5th line for F# should have been chosen so as not to add extra dashes under the 1st line starting from the key of D+.

p.s. pedestrian crossings are in stripes and not in squares because from a distance they would appear as stripes anyway.

Sergio wrote:

The 5th line for F# should have been chosen so as not to place additional dashes under the 1st line starting from the key of D+.

Look, before the convention was created, in some cases (I am talking, for example, of pre-Bach music) the key signature was marked starting from F# in the first space, but upwards, so it always stayed within the staff...

I didn't know it, but I wrote "dovrebbe" [should], anyway now in current music they are descending, there must be a reason for that too.

Since the # is not indicated as a "d", perhaps because it could be confused with the "b" of flat?

Speculations.

I have never explored the subject in this direction; I think that, anyway, the arrangement might also depend on the fact that the 7 accidentals are contained within the musical staff.

Thinking about #: F C G D A E B

as we are normally used to writing them in a score, even though they all use them, they are "contained" within the staff... other solutions would "overflow."

In reality, the issue of staying within the staff is also one of the reasons why the seven-line staff exists... so it makes sense that even accidentals, once the practice of writing the key signature in the clef was introduced, followed this logic.

Speaking of this, as Carlos says, the key signature has not always existed and actually there was also an intermediate phase in which all were written except for the last one. In authors like Zipoli, for example, in pieces in G major, you find only one flat.

As already stated by others, these are conventions that have evolved over time and have progressively responded to the needs of musicians and music.

In the 1900s, the key signature disappeared again. I repeat, means at the disposal of music